1. Field of the Invention
This invention resides in the field of phenolic plastics and more particularly relates to a process for producing a strong, lightweight phenolic foam.
2. History of the Prior Art
Phenolic foams are usually made by catalyzing a liquid phenolic (resole-type) resin which is mixed with blowing agents, such as pentane, and surfactants along with an acid, usually sulfonic. These foams have several problems, including that the remaining acid corrodes objects with which it comes in contact and these foams can spontaneously ignite when their temperatures are elevated to as low as 350 degrees F. Both resole and novolac resins are produced by a polycondensation reaction between phenol and formaldehyde. If the resole resin in its pure form is heated by itself on a platform, it will melt and, over time, cure to a strong, hard sheet. A novolac resin, being made by a formaldehyde-starved reaction, will melt but will remain as a liquid. Phenolic foams are very versatile and have many uses, such as for thermal insulation, fresh flower supports, mine/tunnel uses and for use in making orthopedic foot impressions. These applications are dependent upon the cellular structure of the foam where, for example, a closed cell foam is utilized for thermal insulation while an open-cell foam is used for floral applications. While many phenolic foams are made primarily from liquid resole resins, it is also well known to use novolac resins with chemical blowing agents.
A phenolic foam can also be made by foaming novolac resins using hexamethylenetetramine as a curing and blowing agent. These foams again have serious drawbacks. The decomposition of hexamethylenetetramine during heat cure into formaldehyde and ammonia leaves residual ammonia behind in the foam, making it basic in pH and odorous. Post backing can drive out the residual ammonia but if a closed cell structure is wanted, the outgassing process along with the added temperature hysteresis can be detrimental to the final foam product as well as increasing its production cost.